Add UiEvent logging to volume dialog Events.

Minor refactoring of Events.writeEvent() for testability.

Also added unit test for Events class, covering all 3 types of logging
it does.

Test: atest SystemUITests
Test: mp droid + statsd_testdrive verify UiEvent logging.
Change-Id: Id1cd0e0f5aa9082dab5d85144ea62ed77a36eacd
2 files changed